Thanks to the progress in the scientific and technological fields, various kinds of electronic products have been developed, bringing more conveniences and comfortableness to people's daily life. Memory integrated circuits play a very important role in these electronic products. The currently available memory integrated circuits may be generally divided into two types, namely, volatile memory and non-volatile memory, according to their data storage characteristics.
The non-volatile memory can retain the stored data even when no power is supplied thereto. Among others, a resistive memory is one type of memory device that, after having written in data, would generate a voltage difference for storing the data as 0 or 1. Please refer to FIG. 1 that is a diagram of a resistive memory device. As shown, the RCELL is a main component in the resistive memory device and will switch to a low resistance state or a high resistance state after data is written thereinto. The low resistance is about 20K ohm, and the high resistance is usually higher than 2M ohm. In other words, in the high resistance state, the resistive memory device is almost in the open-circuit (OFF) state. To read the data, a bias voltage can be applied to a terminal of the RCELL, and the current flowing through the RCELL is sensed by a sense amplifier to thereby read out the data. When operating under a normal voltage, the produced cell current is about 4 μA.
Please refer to FIG. 2 that is a diagram of a conventional current-sense amplifier. As shown, in the conventional current-sense amplifier, a P-type metal-oxide-semiconductor field-effect transistor (PMOSFET or PMOS) is connected to form a diode. That is, the drain and the gate of the PMOS are connected to each other. The transistor would have a stabilized gate voltage according to the current flowing therethrough. In this manner, a comparison current can be generated for comparing with the cell current. For example, in the resistive memory, when the current flowing through the transistor in the low resistance state is 4 μA and the current flowing through the transistor in the high resistance state is 0.2 μA, a current of 2 μA can be selected for use as the comparison current (IREF). Since different currents have different stabilized voltages, a simple comparator can be used to compare the cell current with the comparison current and find out the value of data stored in the memory cell.
However, when the conventional current-sense amplifier operates, the required voltages VMAT and VREF must at least be VDD−VTH−VOV. Under general manufacturing process, the PMOS has a VTH about 300 mV. In the case the operating voltage is 0.5V, then the voltages VMAT and VREF are only about 150 mV (i.e. VDD−VTR−VOV=500 mV−300 mV−50 mV=150 mV). Therefore, the conventional current-sense amplifier tends to cause failed memory operation.
It is therefore tried by the inventor to develop a bulk-driven current-sense amplifier and an amplifier operating method capable of increasing the rate of successful operation of memory to meet the current market demands.